1. Field of the Invention
The present invention relates to high performance, electronic motor drives for variable speed or torque control of AC induction motors, and more particularly, to such motor drives which use vector control techniques and velocity feedback.
2. Description of the Related Art
Motor drives are commonly employed to control the application of electricity to a three-phase AC induction motor. Such motor drives include an inverter which switches DC voltage to output lines in a pulse width modulated (PWM) manner to control the frequency and amount of voltage applied to the motor and thus the motor velocity.
Vector control or field-oriented control is one technique used in a motor drive to control the speed and torque of the motor. With this technique, stator current is resolved into a torque-producing, or q-axis, current component Iqs and a flux-producing, or d-axis, current component Ids, where the q-axis current component leads the d-axis component by a 90° phase angle. This type of motor drive also requires knowledge of several motor parameters, such as inductance and resistance of the rotor and stator coils.
For accurate control of a three-phase motor, besides controlling the stator current frequency, it is also necessary to effectively control the slip, which represents the difference between the frequency of the stator current and the electrical frequency of the rotor rotation speed. The slip control is a key component of the high performance motor control to establish an accurate torque control.
U.S. Pat. No. 5,032,771 describes a high performance motor drive which controls the torque, frequency and slip at which the motor operates. The drive includes a torque control loop, a flux control loop, and a frequency control loop that incorporates slip management in response to a voltage difference. The slip is controlled in response to an error between a d-axis reference voltage and a d-axis feedback voltage. Flux weakening is provided in response to an error between a q-axis reference voltage that is sensed when the motor is operating at the base speed and a q-axis feedback voltage that is sensed when the motor is operating above the base speed.
Accurate slip control requires precise information about leakage inductance of the motor. The prior motor drives controlled slip based on an assumed constant value for the leakage inductance. However, the leakage inductance varies due to saturation effects as the motor load increases. Therefore, accurate torque control becomes difficult over a wide torque range when a constant value for the leakage inductance is used for slip control.
Therefore, it is desirable to provide an improved motor control technique that addresses the effects resulting from variation of the leakage inductance.